Kroto et.al. in 1985 (Nature, Vol. 318 no. 14, 1985) discovered that laser vaporization of carbon into flowing helium gas results in formation of a new form of carbon, a compound having the composition C60. The new C60 molecule was named “Buckminsterfullerene”, or “Buckyball” in honor of its almost spheroidal shape, and is a closed truncated icosahedron of carbon atoms, formed from fused five and six-membered aromatic rings.
Subsequent research (Kroto et. al., Nature, 1988) showed that a much larger family of hollow carbon-cage cluster species generically termed “fullerenes” can be generated from carbon-containing substrates. Large closed-cage fullerenes of formulas including C70, C120, C130, C140, C180, and C240 are believed to exist. Other fullerenes with open (i.e., not closed) carbon cages also exist. U.S. Pat. No. 5,876,684 to Withers et.al. is an example of a method and apparatus for producing fullerenes. Other workers have reported formation of fullerenes from vapor phase pyrolysis of napthalene at temperatures of about 1000° C. (see Taylor et.al., Nature, 1993).
Research has also shown (Ebbesen et.al., Nature, 1992) that the family of fullerenes includes hollow graphitic tubules whose dimensions are on the order of nanometers. These hollow graphic tubules have walls formed from sheets of fused six-membered rings. The walls of the nanotubes may comprise many concentric layers of the graphitic sheets, or may have a single layered wall.
Fullerene-type ions and molecules have also been spectroscopically detected in the vapor phase of sooting flames. Fullerenes have also been extracted from soot.
Fullerenes and nanotubes have a variety of uses, including use as superconductors, photo-conductors, micro-lubricants, catalysts, catalyst supports, electrodes for batteries, adsorbents, hydrogen storage media, plant-growth regulators, and pharmaceuticals. In response to such uses, a variety of methods for synthesizing, characterizing and purifying fullerenes and nanotubes have been developed (Srivastava, Energy Sources, 1995). Nevertheless, the yields of fullerenes and nanotubes remain low, and cost of producing and purifying fullerenes remains extremely high, which has significantly limited the commercial viability of many potential applications.
Since the discovery of fullerenes in 1985 by Kroto et al., fullerenes have been the focus of a vast amount of research. One of the fundamental problems with the synthesis of fullerenes is the co-production of soot (i.e., mature soot and/or amorphous carbon, carbonaceous soot) requiring tedious, expensive and wasteful procedures for purification of the product. Much has been done to alleviate this problem for catalytically produced carbon nanotubes. For example, Smalley et al. used disproportionation of high-pressure carbon monoxide over narrowly dispersed catalyst particles to produce reasonably pure nanotubes (Dal et al., 1996). The Smalley et. al. high-pressure carbon monoxide process occurs at the gas-solid interface. More recently, Schlittler et al. (Science, 2001) used thermolysis of nano-patterned layers of C60 and Ni to produce single crystals of nanotubes with identical diameters and lengths. Given the quantity of research on buckminsterfullerene, it is lamentable that little progress has been made to advance the production of C60 without the production of soot or amorphous carbon. The lack of an economic method for producing C60 is especially regrettable in light of Schlittler et al.'s new method for producing single crystals of nanotubes from C60.
The chemistry of flame combustion and the process of soot formation in flames have been investigated. For example, it is known that polycyclic aromatic hydrocarbons (PAHs) are formed in flames, and that the polycyclic aromatic hydrocarbons may be precursors of soot in flames. PAHs are a large class of hydrocarbon compounds having fused five and/or six membered aromatic ring residues. A list of about 622 known polycyclic hydrocarbons has been tabulated by Sanders and Wise of the National Institute of Standards and Technology, in NIST Special Publication 922, available at inter-alia, the NIST website.
Polycyclic hydrocarbons initially and predominantly grow in the vapor phase of a flame by step-wise condensation of two carbon fragments. PAH compounds with even numbers of carbon atoms, comprising planar arrays of fused six-membered benzene residues are believed to predominate over PAH compounds with odd numbers of carbon atoms or five-membered rings in flames, because of differences in thermodynamic stability.
Baum et.al. in 1992 suggested that fullerenes may form by coagulation or condensation of PAH molecules with other PAH molecules or immature soot particles.
For the past fifty years and more, theories of soot formation have been based on the chemistry of the gas phase. The most probable reason for this approach is that most hydrocarbon precursors are gases at the temperatures where soot is formed. However, there is a form of hydrocarbon that does exist in the condensed phase under these high temperature conditions called precursor soot. Unfortunately, the very existence of this material has been shrouded in controversy and has resulted in many strange ideas for the process of soot formation.
Though reference cannot be found in the literature, the controversy likely stems from the composition of precursor soot. In general, the composition of precursor soot can be described by two extractable fractions, the aromatic and tar fractions. The aromatic fraction is primarily composed of free polycyclic aromatic hydrocarbons (PAHs) as defined by chromatographic and mass spectrometric analysis. The tar fraction is composed of aromatic and aliphatic hydrocarbons as defined by spectroscopic analysis (IR and NMR). The controversy arises from the physical properties of the free PAHs; their boiling points are much lower than the temperatures where precursor soot is formed. Since no species can physically condense when its equilibrium vapor pressure is greater than the pressure of the surrounding environment, it can incorrectly be concluded that this oil only condenses during the sampling process. Consequently, unsupported and mostly unpublished explanations for the existence of precursor soot abound.
Evidence of precursor soot in flames was observed and reported by Parker and Wolfhard in 1950. Definitive evidence for the existence of precursor soot was demonstrated by the carbon black industry as reported in 1956 by Sweitzer and Heller. They observed a white mist (micron-sized particles) develop in their furnace. Sampling and chemical analysis of the white mist showed it to be an oil of PAHs. Further heating of the white mist produced soot. Their work proved that a high-temperature stable form of liquid hydrocarbon (precursor soot) could be produced by fuel pyrolysis.